1. Field of the Invention
The present invention relates generally to a camera and method of use for holography and holographic interferometry, and more particularly relates to an apparatus and method for the recording of a continuous sequence of high resolution holograms using a photosensitive photopolymer material such as bacteriorhodopsin (bR) to achieve a high speed, high resolution series of holographic recordings.
2. Description of Prior Art
Holographic nondestructive testing (HNDT) is a well known technique. According to this method, an interferometric fringe pattern is produced by one of three methods: by superimposing two recorded holograms (double exposure interferometry), by allowing a previously recorded hologram to be visually superimposed onto a real time image of an object (real time interferometry), or by recording a single hologram during the vibration of an object (time average interferometry). According to the first two of these methods, a reference hologram is recorded. The object is then stressed by thermal, pressure, or by other means and the reference hologram is interfered with a second hologram or with a real time image of the object in the stressed state. The overlap of the reference hologram with the subsequent hologram or real time image produces an interferogram composed of a fringe pattern. The fringe pattern contains information on the structural integrity of the object. In the third method, where a hologram in recorded of a vibrating object, the hologram records light and dark areas depicting the nodes and anti-nodes, respectively, of the vibrating object. These bright and dark areas give information on the structure of the object as well.
Several prior art devices are in use. The main disadvantage of these devices is their inability to record holograms in a continuous sequence, at high resolution, and without the necessity of a development cycle. Mother important disadvantage is that the materials which form the basis of the most common HNDT instruments are of fixed geometry, and cannot be adapted to customized applications. A list of these materials is given in Table 1 below.
TABLE I __________________________________________________________________________ Comparison of the Most Common Recording Materials Used in HNDT Diffraction Material Duty Cycle Sensitivity Resolution Efficiency Notes __________________________________________________________________________ Silver Halide N/A 50 5000 &lt;2 Requires wet processing, non-erasable, $5/plate Thermoplastic 30 s 10 800 0-10 Electronic processing, high voltage required, noisy images Charge Coupled .mu.sec-msec 10.sup.-3 80 N/A Interferograms possible only Device (CCD) through comparison of single Arrays frames. Double exposed images not possible. Photorefractive msecs-secs 10.sup.6 Very high &lt;40 Limited by crystal geometry. Crystals Very expensive. Self- developing, $10K/crystal. Bacteriorhodopsin .mu.sec-hrs 250 @ 5000 &lt;7 Thermal, chemical lifetime optical control. Self developing. density = 2 Flexible geometry, $200- $1000/plate __________________________________________________________________________
Silver halide plates have been long outdated for most applications of HNDT. Though silver halide materials have a high resolution of 5000 lines/mm, they are one-shot in nature, so that they cannot be erased and reused to record subsequent holographic exposures. A silver halide plate requires off-line wet-processing, so that after exposure, it must be removed from the holography system, developed, and then later reinserted into the same or similar holography set-up for reconstruction. When a silver halide plate is used to perform real time interferometry (the second method detailed above), the plate must be reinserted in the original system to within a fraction of the wavelength of light from its original position. In addition, if a plate does not dry uniformly after development, the result is a noisy image. Also, silver halide plates are not panchromatic, so their use is restricted to a range of laser wavelengths.
By far, the most popular material for recording holograms for performing HNDT is the thermoplastic material, most commonly used in the HC300 Thermoplastic Camera developed by Newport Corporation, Irvine, Calif. The development of this camera made it possible to perform HNDT without the inconvenience of removing and wet-processing silver halide plates from the optical system. The camera functions on the basis of a 35 mm thermoplastic slide, with which it is possible to record, develop, and then erase a hologram electronically, without removal of the slide. Thermoplastic material can be used to perform HNDT by the second and third methods mentioned above (real time and time average interferometry). However, it is very difficult to superimpose two holograms on the material, so that the first method of double exposure interferometry is rarely used in this context.
The major disadvantage of the thermoplastic camera when used for HNDT is that it requires a 30 second development cycle which strictly limits its use in applications requiring a rapid sequence of interferograms. In addition, its resolution is only 800 lines/mm. The diffraction efficiency of a plate decays exponentially over the guaranteed lifetime of 300 shots, so that the image quality and fringe contrast gradually fade. In industrial environments the plates are particularly prone to attract dust through the use of both corona electrons in the development cycle and forced air to cool the plates after erasure. Also, the use of high voltage in the development and erase cycles often causes breakdown of the thermoplastic material. These factors result in intefferograms with relatively poor signal to noise ratio.
One method that avoids the time delay associated with a material development cycle is known as Electronic Speckle Pattern Interferometry (ESPI), or TV Holography. An example of the latter technique is described in U.S. Pat. No. 3,816,649. In ESPI, a charge coupled device (CCD) camera is used to capture a succession of single holograms, at the standard video rate of 30 frames/sec. Interferograms are formed by software manipulation of pairs of holograms. While ESPI makes it possible to view a fringe pattern directly with a video camera, it has important disadvantages.
Because ESPI is CCD-based, the resolution of the technique is limited by the size of CCD pixels, which are large when compared to the fundamental recording elements of the other materials such as the grains of developable silver found in silver halide materials. This causes the resolution of a single ESPI hologram to be only 80 lines/mm, which is very coarse in comparison with silver halide. When this single low resolution hologram in overlapped in software with another low resolution hologram, the result is an extremely low resolution interferogram. The result is that many of the finer details of the resulting fringe pattern are lost, limiting the use of ESPI in HNDT.
Photorefractive crystals have demonstrated great potential in optical computing and beam steering architectures. However, they have some notable shortcomings which must be taken into account in any applications requiring real time processing and analysis. Photorefractive crystals form diffraction gratings by the migration of spatial charge on exposure to light. While this means very high resolution, it can be an extremely slow process. In one study of iron-doped lithium niobate, maximum diffraction efficiencies of 39% were achieved only after 10 minutes, at a flux of about 25 mW/cm.sup.2. In cases where the photorefractive material is faster, it may require a very large amount of light to generate the required densities. For example, at 1 W/cm.sup.2, barium titanate has a response time of tens of msecs. These time and sensitivity scales are obviously far outside the boundaries of real time HNDT abilities. In addition, photorefractive crystals are very expensive and not very versatile, due to their crystalline geometry. A 1 cm.sup.3 of lithium niobate can cost up to $10,000, for example. For these reasons, photorefractive crystals have never been incorporated into a practical instrument for use in HNDT.
Thus, there is a need for an apparatus that overcomes the disadvantages of prior art devices using silver halide, thermoplastic, and other such instruments for HNDT. The present invention overcomes the disadvantages of the prior art described above and provides other related advantages detailed in the following summary of the invention.